1) Field of the Invention
The present invention relates to a drive control method and apparatus that controls the drive of a pulse motor so that a rotor driven by the pulse motor rotates at a constant angular velocity or a movable element driven by the pulse motor moves at a constant velocity, and to a belt device, an image forming apparatus, an image reading apparatus, and computer products.
2) Description of the Related Art
A drive control method disclosed in Japanese Patent No. 2754582 is a method of detecting an angular velocity of a transfer roller based on an output pulse signal of an encoder provided on a shaft of the transfer roller as a rotor. In this method, the drive of a pulse motor that rotates the transfer roller is controlled so that a detected value of the angular velocity coincides with a preset target value of the angular velocity.
A drive control method disclosed in Japanese Patent Application Laid Open No. 2000-047547 is a method of detecting an angular velocity of a driven roller based on an output pulse signal of an encoder provided on the driven roller as the rotor. The driven roller is turned by a drive roller through a belt that is wound around between the driven roller and the drive roller. In this method, the drive of a pulse motor that rotates the drive roller is controlled based on a detected value of the angular velocity and a preset target value of the angular velocity. By controlling the drive of the pulse motor based on the results of detecting the angular velocity of the rotors such as the transfer roller and the driven roller in the above manners, the rotors are controlled so as to rotate at a constant angular velocity.
Conventionally, in order to keep the drive status of a motor driver appropriate, a displacement is measured using an encoder to perform feedback control on the motor driver based on the result of measurement. A relation between the resolution of the encoder or a marker sensor and the magnitude of a variation component of a target to be controlled has been generally recognized by persons skilled in the art as follows. That is the displacement can be measured only after the encoder having the resolution finer than the double amplitude of the variation component of the target, is used. Moreover, in order to obtain practical measurement accuracy, it is desirable to use an encoder having the resolution sufficiently fine with respect to the variation of the target.
Therefore, the encoder or the marker sensor having the resolution sufficiently finer than the double amplitude of the variation component of the target has to be used. Because the resolution affects the image quality of a formed image or the reading accuracy unless a drive system to be used for conventional image forming apparatuses and image reading apparatuses is controlled accurately.
As explained above, in the drive control methods described in the above-mentioned patent documents, the angular velocity of the rotor such as the transfer roller and the driven roller is detected based on the output pulse signal of the encoder. The drive of the pulse motor is controlled so that the detected value approaches the target value of the angular velocity. When the drive of the pulse motor is controlled based on such a detected value, it is possible to control so that an error in the angular velocity of the rotor between control timings is reduced. However, the error cannot be eliminated perfectly. This is because if an error occurs in the angular velocity of the rotor, an error in an angular displacement of the rotor caused by the error in the angular velocity is cumulatively increased. Such an error in the angular displacement of the rotor causes the variation in an image size in the image forming apparatus that includes a photosensitive drum and a transfer roller as the rotor. The error may cause color misalignment when the color image forming apparatus that obtains a color image by superposing images of colors on one another.
There is a case where the control is performed so that a movable element driven by the pulse motor linearly moves at a constant velocity but not performed so that the rotor is rotated at a constant angular velocity. In this case, the velocity of the movable element is detected based on an output pulse signal of a sensor that detects a linear movement of the movable element. The drive of the pulse motor is then controlled so that a detected value of the velocity approaches a target value of the velocity. Even if the control is performed in such a manner, the error in the displacement of the movable element may be accumulated in the same manner as the case where the rotation of the rotor is controlled.
Furthermore, in such a conventional drive control apparatus, the encoder or the marker sensor, whose resolution is finer than the double amplitude of the variation component of the target to be controlled at all times, has to be used, which causes the cost of the high resolution encoder and marker sensor to be inevitably increased. Thus, the cost of the image forming apparatus and the image reading apparatus with the devices is increased naturally.